Local corrosion rate

In all cases of localized corrosion, tlie ratio of the catliodic to tlie anodic area plays a major role in tlie localized dissolution rate. A large catliodic area provides high catliodic currents and, due to electroneutrality requirements, tlie small anodic area must provide a high anodic current. Hence, tlie local current density, i.e., local corrosion rate, becomes higher witli a larger catliode/anode-ratio. 

Multiinformational Prohes Corrosion probes can provide more information than just corrosion rate. The next three types of probes yield information about the type of corrosion, the kinetics of the corrosion reaction, as well as the local corrosion rate. 

The corroded tubercle floor is almost always a dish-shaped depression, much wider than it is deep (Fig. 3.23). Undercutting is very rare. The metal-loss width almost exactly matches the tubercular mound width. Corrosion rates exceeding 50 mil per year are rare, except when tubercles are young. Average local corrosion rates are usually 20 mil per year or less. 

Cooling water pipes are essential for the operation of power stations and must not cease to function. Pipelines for fire fighting are also important for safety reasons. Such steel pipelines are usually well coated. At areas of unavoidable damage to the pipe coating, there is an increased danger due to cell formation between steel and concrete where local corrosion rates of >1 mm a are to be expected [4], Damage to pipelines for fire fighting has frequently been observed after only a few years in service. 

That is, to determine the correct corrosion rates in pitting corrosion, as shown in Fig. 37, it is necessary to know the local corrosion currents on the electrode surface. The corrosion current observed is, however, obtained as the total current, which is collected by the lead wire of the electrode. From the usual electrochemical measurement, we can thus determine only an average corrosion current (i.e., the corrosion rate). Hence if we can find some way to relate such an average rate to each local corrosion rate, the local corrosion state can be determined even with the usual electrochemical method. 

If the flaw in the passive film is smaller in cross section and greater in depth, then with reference to Fig. 7.6, the resulting increase in resistance can lead to an (IR)2 potential drop that decreases the potential in the bottom of the flaw and/or pit to E2. Then passivity cannot be maintained, and the corrosion current density increases to i2 in the active range. The local corrosion rate is much higher, and a stable pit is initiated at the much higher current density. When the pH of the bulk envi-... 

The calculation of a local corrosion rate (penetration rate) is intrinsically difficult because the area of the localized attack is not known [39]. 

The droplet height varied between 400 and 1100 pm. The local corrosion rates were determined by EIS and electrochemical polarization measurements. An increase in corrosion rate was observed with decreasing electrolyte thickness below 800 pm. The increase of the corrosion rate was due to the decrease of the diffusion layer thickness, resulting in an increase in oxygen reduction rate. 

The floors were attacked by microorganisms. Shallow pits were formed on the floor after 25 years of service. Localized corrosion rates reached up to 0.5 mm/year. Heterotrophic bacteria and sulfate-reducing bacteria (SRB) were found in the sludge. 

Under-deposit attack or poultice corrosion may occur when a metal is locally covered by foreign, absorbent (organic or inorganic) materials [40,45]. In this case, attack can proceed even when the bulk of the system is dry due to retention of moisture in the poultice. The corrosion mechanism is similar to crevice corrosion in that the deposits act to limit the migration of oxygen to the covered area. This leads to acidic shifts in pH, concentration of Cl ions in the shielded area, and a shift to a more active corrosion potential under the deposit. Local corrosion rates can be very high due to the large cathode-to-anode area ratio. 

An example of shallow-pit corrosion is given in Fig. 1-34. This corrosion damage was observed in an open recirculation cooling system after 18 months of operation. The tube, made of structural steel (St 37), showed several local attacks by pitting with a local corrosion rate of up to 3 mm year" . Iron sulfide was detected in the corrosion products. This indicates the presence of anaerobic sulfate-reducing bacteria (SRB). Under strictly anaerobic conditions the bacteria reduced the sulfates to sulfides leading to enhanced corrosivity of the medium (Weber and Knopf, 1994). 

Because the electrode surface area is usually between 1 and 0.03 mm, which is approximately 2 to 4 orders of magnitude less than that of a typical LPR probe or a typical electrochemical noise (EN) probe, the prediction of penetration rate or localized corrosion rate by assuming uniform corrosion on the small electrode is realistic in most applications. CMAS probes have been used for monitoring localized corrosion of a variety of metals and alloys in the following environments and conditions ... 

General and local corrosion rate determined non-destructively in 3D by SR liCT... 

Variously designed weight-loss coupon, electrochemical and surface analytical techniques have been utilized in REM-based corrosion inhibitors and conversion coatings research. In particular, electrochemical techniques including EIS and polarization measurements have been widely used to evaluate corrosion inhibition by REM compounds under various environmental conditions. Relatively less attention has been paid to the evaluation of localized corrosion inhibition by REM-based compounds, probably because of methodological difficulties and complexities in making accurate localized corrosion rate measurements. Recently developed techniques such as the scanning probe techniques, electrochemical noise analysis and the wire beam electrode are expected to be useful tools in further REM inhibitor research. 

Y. J. Tan, Monitoring Localized Corrosion Processes and Estimating Localized Corrosion Rates Using a Wire-Beam Electrode , Corrosion, 54,403 (1998). 

---